This invention concerns a multiphase polymer blend which is tough and flexible. More particularly, it concerns such a blend in the form of particles of harder polymer in a continuous matrix of softer polymer.
Flexible finishes can be grouped in two broad classes, acrylic-containing and nonacrylic. This invention relates to flexible finishes containing a substantial amount of acrylic resin, by far the smaller group of materials because of the well known observation that acrylic polymers are generally hard and brittle, or soft and weak.
The advantages of acrylic polymers with respect to outdoor durability, ease of manufacture and relatively low cost continues to make it desirable to seek routes to use them in flexible finishes. Two approaches have emerged in the past, blending and modification by grafting for example to make acrylourethanes, as in U.S. Pat. Nos. 4,208,495, 4,208,494, 4,143,091, 4,034,017, 3,975,457, and U.S. Pat. No. 3,919,351. The grafting approaches also include modification of a hydroxy acrylic with caprolactone to produce hydroxyester side chains, A. N. Theodore and M. S. Chattha, J. Coatings Technology, vol. 54, #693, pp. 77-81 (1982).
Previous attempts to achieve flexibility by blending of acrylic polymers have involved crosslinked blends of two acrylics of moderate molecular weight, for example U.S. Pat. No. 3,773,710--Victorius (Nov. 20, 1973) or in crosslinked blends of low molecular weight polyester with moderate molecular weight acrylic polyol in U.S. Pat. No. 4,076,766--Simms (Feb. 28, 1978). In this latter patent the concept of improving the low temperature flexibility of the film by using a low glass transition temperature polyester polyol coreactant with the acrylic polyol is exploited, but the resultant films lack hardness. This is thought to be due to the selection of a relatively low molecular weight acrylic polyol for the coating to avoid application problems such as web formation seen when high molecular weight acrylic polymers are used in coatings, and to achieve more than 20% by weight application solids. This causes incomplete or no phase separation upon crosslinking. Even if molecular weights were selected to give better phase separation, application properties are not as good as might be desired. A high degree of phase separation is now thought to be necessary for maximum hardness with flexibility in these blends, but concomitant with phase separation often is haze or loss of transparency which can vitiate a product for various purposes.
Other work on tough, flexible finishes which can be used both on metal and flexible plastic or rubber is illustrated by U.S. Pat. No. 4,548,998 on combinations of polyurethanepolyol and a curing agent, U.S. Pat. No. 4,545,132 on polyesterpolyol and curing agent, U.S. Pat. No. 3,882,189 on polyesterurethanepolyol and curing agent, U.S. Pat. Nos. 3,954,899 and 3,962,522 on polyester, polyurethanepolyol and curing agent, and U.S. Pat. No. 4,419,407 on polyesterpolyol, polyurethanepolyol and curing agent. While these inventions may produce tough, flexible finishes, they neither have the functional advantages of acrylic-containing finishes nor provide an optimum balance of mechanical properties.
U.S. Pat. No. 4,180,613--Vissiliou (Dec. 25, 1979), discloses the use of tough and relatively sticky particles in a hard matrix of silicone resins to minimize crack propagation and permit making a thicker coating without mud crack formation on drying.
U.S. Pat. No. 4,468,492--Piccirilli (Aug. 28, 1984), is another example of using a relatively hard matrix or film-forming phase with a soft second phase blended in, e.g., acrylic lacquer film formers with a T.sub.g above 25.degree. C. blended with siloxane-substituted materials with a T.sub.g below -5.degree. C.
Group transfer polymerization is established as a useful technique for preparing acrylic polymers of prescribed architecture and narrow molecular weight. It is particularly useful in making multi-armed stars with narrow-polydispersity of molecular weight of the arms themselves and of the stars with attached arms. See. U.S. Ser. No. 627,913 and 627,919, both filed July 5, 1984, and related PCT International Publication WO86/00626--Spinelli, published Jan. 30, 1986, and the references cited therein.
The patents, applications and articles cited above are incorporated herein by reference for a fuller disclosure and understanding of the state of the art and the background of this invention.
Enamel paints with an acrylic resin and an acrylic star polymer (without OH groups in the arms) are disclosed in Ex. 23, and stars with hydroxyl-containing arms are disclosed in Ex. 2 of the WO publication.
The resulting enamel would not have the degree of toughness and flexibility needed for use in a unitary clear coat over both base-coated metal and rubber parts of an automobile.